(a) Technical Field
The present invention relates to activation of a fuel cell stack. More particularly, it relates to an apparatus and method for activating a fuel cell stack, which significantly reduces the time required for activation and the amount of hydrogen used for the activation by employing a vacuum wetting process in a shutdown therein.
(b) Background Art
There has been an increasing request worldwide for the development of environmentally-friendly vehicles due to growing public concerns and interests on environmental issues and CO2 regulation. Therefore, environmentally-friendly and high efficiency fuel cell vehicles, which can replace internal combustion engine vehicles which cause environmental pollution, have attracted a lot of public attention.
In particular, in a fuel cell vehicle, after assembly of a fuel cell stack which is a core component of the fuel cell vehicle, a stack activation process is performed to (1) ensure a three-phase electrode reaction zone is present, (2) remove impurities from a polymer electrolyte membrane or electrodes, and (3) improve the ionic conductivity of the polymer electrolyte membrane. By ensuring these three qualities, the fuel cell stack is able to exhibit normal performance characteristics.
In particular, during initial operation of the fuel cell stack, its activity is reduced in an electrochemical reaction, and thus it is necessary to perform a stack activation process in order to maximize the normal initial performance. This stack activation process is also called pre-conditioning or brake-in, and its purpose is to activate a catalyst which cannot participate in the reaction and to ensure a hydrogen ion passage by sufficiently hydrating electrolytes contained in the electrolyte membrane and electrodes.
FIG. 1 shows an example of a conventional stack activation method/process using a pulse process which includes a high current density discharge and a shutdown discharge. As shown in FIG. 1, the conventional stack activation method is configured to perform a pulse process which utilizes a high current density discharge and a pulse discharge in a shutdown state, repeatedly, several to dozens of times. However, this conventional stack activation method requires a processing time of about 1.5 to 2 hours with respect to a 220-cell sub-module, for example.
In more detail, according to the conventional stack activation method, a high current density (1.2 or 1.4 A/cm2) is discharged for 3 minutes and a pulse discharge process performed during a shutdown state for 5 minutes. These two processes are repeatedly performed about 11 times in all before the activation process is complete.
However, the amount of hydrogen used in the activation process through the pulse discharge increases as the processing time increases. That is, the conventional activation method, in which the pulse discharge is used in a shutdown state, can vary the flow of water in the fuel cell stack, thereby increasing the activation rate. However, the time required for the activation is about 105 minutes and the amount of hydrogen required for the activation is about 2.9 kg with respect to a 200-cell sub-module, for example. As a result, the method requires a long processing time and a large amount of hydrogen to be effective.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.